Various solutions have been disclosed addressing the problem of aircraft doors that had accidentally not been properly closed and locked at take-off. Such doors can be unintentionally opened during flight, for example, by being blown open by a pressure differential created between the higher pressure in the cabin and the lower pressure outside the aircraft. The previously suggested solutions involve a door monitoring system that is installed in an aircraft to detect and evaluate information regarding the status of the aircraft doors. The prior art system then provides the detected and evaluated information as a visual indication on the aircraft monitoring system in the cockpit of the aircraft. In order to preclude the cockpit crew from performing incorrect actions as a result of erroneous interpretations of the cockpit indicator or as a result of an erroneous indication, additional mechanical means are provided. Specifically, mechanically operatable air vent flaps are arranged in each door of the aircraft in a manner to be operatively interconnected with the closing and locking mechanism of the door. Through the operative interconnection with the closing and locking mechanism of the door, the air vent flaps are activated only when the respective door is not properly closed and locked. In that case, the spring loaded air vent flap of the unlocked door opens when a predetermined over-pressure is achieved within the aircraft fuselage. That is to say, once a predetermined pressure is achieved within the aircraft fuselage relative to the lower pressure outside the aircraft, the air vent flap opens and allows air to escape from the aircraft to achieve a pressure compensation between the interior of the fuselage and the outside environment around the aircraft. This prevents proper pressurization of the aircraft from being achieved and therewith prevents the improperly closed and locked door from being blown out of the aircraft fuselage by the pressure differential that would exist if the aircraft were properly pressurized.
The above described prior art electronic systems involve a substantial disadvantage in that the cockpit crew is necessarily required always to take the correct and appropriate actions based on the cockpit indicators, to avoid an excess pressurization of the interior of the aircraft when a door is not properly closed and locked. Otherwise, all persons on board the aircraft would be endangered if the required pressure compensation were not initiated. In order to avoid this critical situation, and to guarantee the safety of all passengers on board the aircraft, the previous electronic indication systems must be used in conjunction with the above described mechanical air venting flaps, which are functionally installed in each door in a known manner.
Thus, the above described prior systems have a further disadvantage in that it is necessary to equip each door and access hatch in the aircraft fuselage with the above described air venting flaps. Furthermore, the operation and function of each respective air venting flap is dependent upon an additional mechanical linkage, including diverse lever linkages. Thereby, the separate operation and activation of each air venting flap in each aircraft door and access hatch increases the effort and complexity involved in operating each door and also increases the weight of each door and therewith the weight of the entire aircraft. Because each air venting flap must operate reliably, additional maintenance procedures are required to preclude, with a great degree of assurance, the danger of a blockage or operating failure of any one of the air venting flaps as a result of wear defects which are unfortunately exasperated by the additional effects of corrosion.
Conventional door monitoring systems to be employed in aircraft are disclosed, for example, in the European Patent Publication 0,321,994 and in U.S. Pat. Nos. 4,915,326 and 4,552,326. All three of the proposed solutions for the problem disclose a system in which the aircraft doors are locked as a result of an internal pressure increase in the aircraft or other parameters. According to U.S. Pat. No. 4,552,326, the door is to be locked mechanically be means of a locking bolt or pin that is operated by means of a pneumatic actuating system. The European Patent Publication 0,321,994 and U.S. Pat. No. 4,915,326 each disclose a system in which the door is locked by locking pawls that are electrically activated. Thus, in the disclosed systems, a pressure is built up in the aircraft, which realizes the locking of the aircraft doors, but simultaneously prevents opening of the doors. That is to say, the disclosed solutions do not allow the thusly operated aircraft doors to be opened in the state that is locked due to the increased pressure. The prior systems disadvantageously do not consider and provide a proper control of pressurization to allow a varied movement of the aircraft door or doors from a locked state into an opened state, nor the regulated preventing of a pressure build-up within the aircraft fuselage.